1. Field of the Invention
The present invention relates to a method of producing highgloss coated papers with heated calendering apparatus through the use of coating compositions comprising pigments and synthetic polymeric latices.
2. Description of the Prior Art
High-gloss coated papers can be produced by the application of an aqueous coating composition containing pigment and thermoplastic binder and subsequently developing gloss in the coated paper by means of hot calendering, such as by supercalendering or gloss calendering. A supercalender comprises a "stack" of alternating steel calender rolls and resilient backing rolls, with means for heating the steel rolls or heat being generated by friction. Usually the paper web is threaded through the stack, thus wrapping the heated roll for a considerable portion of its circumference. Alternatively, the paper web may be passed directly through the nip formed by the heated roll and the backing roll by the use of fly rolls. The use of a supercalender involves relatively high nip pressures, generally in the range of 1000 to 2000 pounds per linear inch (pli) and typically 1200-1600 pli. The temperatures of the steel finishing rolls are generally 140.degree. to 180.degree. F. While supercalendering suffers from the disadvantage of compacting and densifying the coated paper web during the calendering operation, it has the advantage of greater speed and lower cost of equipment relative to gloss calendering which is almost invariably done in-line with the coating operation.
The method of finishing coatings on paper by means of hot calendering known in the paper-coating art as "gloss calendering" involves the production of a glossy surface on paper or related web materials by contacting the surface of a coated substrate with a polished finishing drum under temperature conditions sufficient to cause a temporary condition of plasticity in the surface to thereby obtain a high degree of finish or gloss without unduly compacting the substrate. This higher bulk compared to that obtained by supercalendering leads to increased brightness and opacity, which are desirable properties in coated printing papers, and permits the use of a lighter basis weight paper to provide a given caliper. In gloss-calendering of paper, an aqueous coating composition is applied to a paper web, the web is calendered in the nip formed between a gloss-calender drum and the resilient backing roll and the paper web is removed from contact with the drum as it emerges from the nip. The pressure conditions in the gloss-calender nip are generally lower, e.g., 500-900 pli, than in the supercalender, and the temperature conditions are typically higher --275.degree.-350.degree. F.
Aqueous paper coating compositions generally comprise a mineral component, which is preferably predominantly clay, but may also include other mineral pigments such as titanium dioxide, zinc sulfide, or calcium carbonate, and a thermoplastic binder. It has recently been suggested (in U.S. Pat. No. 3,583,881 granted June 8, 1971) that so-called "hard" polymers be employed as the binder material in a paper coating composition in order to obtain a high gloss coated paper product. By "hard" is meant that the thermoplastic polymer has a relatively high apparent second order transition temperature or inflection temperature, more commonly in the paper coating art called the glass transition temperature (T.sub.g), which may be found by plotting Young's modulus of rigidity against temperature.
Each polymer has its own "glass transition temperature" (T.sub.g); this term is well known in the art and is generally used to define or describe a temperature above which the polymer has acquired sufficient thermal energy for molecular rotational motion or considerable torsional oscillation to occur about the majority of bonds in the main chain. This term is also used to define a "minimum film forming temperature" of polymer latices above which the polymer particles are capable of being coalesced by surface tension upon evaporation of water to form a film. In effect, then, the term "glass transition temperature" or "minimum film forming temperature" describes a type of internal "melting" point for polymer latices, but not a phase change, at and about which the polymer preserves the outward appearance of a solid but at the same time behaves more like a viscous liquid in its ability to undergo plastic flow and elastic deformation. For the purposes of this invention, the term "glass transition temperature" may be used interchangeably with and defined as the "minimum film forming temperature" of a polymer latex. In actuality, this "transition" occurs over a small range of temperatures rather than an exact point.
From this definition it may be seen that if the temperature T is taken as room temperature (68.degree. to 77.degree. F or 20.degree. to 25.degree. C), then any polymer having a Tg substantially greater than T, for example, 90.degree. F or 32.degree. C, will be a non-film former at T, while any polymer with a Tg substantially below T, for example 60.degree. F or 16.degree. C, will be a relatively good film former at T. In the art to which the present invention pertains, the expression "hard" refers to binders having a Tg substantially greater than room temperature and generally a Tg in excess of 100.degree. F or 38.degree. C.
It is stated in the above-mentioned patent U.S. Pat. No. 3,583,881, that in order to prevent sticking of the coating to the hot steel roll used in the gloss calender, the mean Tg value of all polymer components of the binder must be at least 43.degree. C. It is further stated that the drying of the coating is effected at an elevated temperature to assure fusion of the polymer therein, the temperature being about 20.degree. to 60.degree. C higher than the Tg of the polymer incorporated in the coating.
Another U.S. patent which is directed to the production of high-gloss papers by coating the papers with a composition comprising a pigment and a "hard" binder, U.S. Pat. No. 3,634,298 granted Jan. 11, 1972, presents a theoretical explanation as to why "hard" binders promote gloss. The pigment is oriented during the drying cycle by surface tension effects which bring the pigment matrix to minimum volume. This is also the point of maximum orientation and highest gloss in the unfinished condition. Binders, however, freeze the pigment in a random orientation as soon as pigment binding occurs, which prevents maximum orientation and gloss before calendering. This binding action commences when the emulsion is broken in the drying cycle and film formation occurs. As the minimum film forming temperature is increased, the time until coalescense in drying is increased, thereby increasing the time for orientation to occur. By this reasoning, gloss is a function of the glass transition temperature.
As will become apparent hereinafter, such a conclusion is only partially correct. Both of the above-mentioned patents, moreover, teach that the binder be coalesced during the drying cycle in order that it holds the pigment in place. U.S. Pat. No. 3,583,881, column 4, lines 18 through 22; U.S. Pat. No. 3,634,298, column 1, lines 69 through 72.